Metal detectors are popularly used by hobbyists and collectors to search for buried or otherwise hidden metal objects of value or particular interest, such as coins, jewelry, and artifacts of historical significance. Metal detectors typically employ a time-varying electromagnetic field to “interrogate” a target volume of ground, proximate the detector, that may contain such objects. A metal object in the ground receives electromagnetic energy from the field and responds by modifying the field according to the electric and magnetic properties and quantities of the materials of which the object is composed. By consulting a look-up table of the responses that would be expected from various types of objects, a response can be used to characterize or identify, as well as detect, the object.
In addition to valuable metal objects of interest, the ground often contains a substantial background of relatively valueless objects such as soft drink cans and pull-tabs, and the ground itself is often composed of metallic compounds, particularly compounds containing iron. This background also responds to the interrogating field, and it is therefore necessary for the detector to be able to distinguish between objects of interest and the background. As would be expected, it is always highly desirable to improve the detector's capabilities in this regard.
Traditionally, metal detectors have employed a single interrogating frequency. Electromagnetic radiation at the selected interrogating frequency is broadcast or directed to a target volume of ground. The detector measures a response which is an electromagnetic signal which has a magnitude and phase that are in general altered from the magnitude and phase of the original radiation as a result of the (complex) impedance of the target.
The impedance of the target has a real part that produces a (vector) component of the response that is in-phase with the interrogating frequency, and a so-called imaginary part that produces another component of the response that is in-quadrature, or 90 degrees out of phase with the interrogation frequency. It may be noted that the real component of a vector is often identified in engineering and mathematics with the horizontal “x” axis of a standard Cartesian coordinate system, while the imaginary component is identified with the vertical “y” axis. However, in the metal detector art, metal detector responses are sometimes graphed so that the real part of the response is plotted on a vertical “Y” axis that represents zero phase shift, with a horizontal “X” axis depicting negative and positive phase deviations from the “Y” axis. This scheme is used herein.
The real, or “Y” component of a given frequency specific response vector represents the effect of the conductivity of all of the material contributing to the response at the specific frequency, while the imaginary, or “X” component represents the effect at the frequency of the reactance of this material. For non-ferrous metal materials and saltwater, the resistive component of the response will be much greater than the reactive component; conversely, for ferrous metals and soil containing iron, the reactive component is larger than the resistive component. The detector resolves the total frequency specific response into its Y and X components, each providing information about the target volume of ground that can be used to advantage in discriminating between objects in the ground and the background.
More recently, metal detectors have been provided that employ two interrogating frequencies, so that four response components may be obtained as described above. A lower frequency is provided that is particularly suited for detecting larger objects, especially those of good conductors like copper or silver, and a higher frequency is provided that is more suited for detecting smaller objects and objects that are composed of metals which are relatively poor conductors. The user of such a detector may select between the two frequencies depending on the type of object that the user is searching for.
To subtract out the effect of the ground on the response, metal detectors typically provide a feature known as “ground balancing.” In single frequency detectors, ground balancing is conventionally achieved by selecting a location on the ground for calibrating the detector, and determining how to linearly combine the X and Y components of the response so that the response is zero, or put another way, how much to rotate the X and Y coordinate system so that, at the phase angle of the ground, the response is nulled. In practice, this can be achieved by varying the phase angles of respective synchronous demodulators so that the demodulators are insensitive to components with a phase equal to the phase angle of the ground.
Similarly, for dual frequency detectors, Candy, U.S. Pat. No. 4,942,360, proposes ground balancing by forming various linear combinations of the four X and Y components. For example, to null the response for reactive soil, the '360 Patent proposes among other things forming a linear combination of the reactive components for the two frequencies.
In summary, dual frequencies have been used, as explained above, to permit arbitrary selection between the frequencies in order to tailor the frequency to an anticipated object, and the response components for the two frequencies have also been used in combination to effect ground balancing. However, metal detectors have heretofore not made full use of the information present in the response as a result of interrogating a target volume of ground with multiple frequencies. Particularly, prior art metal detectors have not employed the full benefit of the information present in a multiple frequency response to improve the capability of the detector to distinguish between metal objects. Accordingly, there is a need for a method and apparatus for distinguishing metal objects employing multiple frequency interrogation that provides for improving the capabilities of a metal detector in this important regard.